Electric RF multipole fields have long been used to guide ions in ion guides without the use of magnetic fields. These RF multipole fields can easily be generated with at least two pairs of long, thin, parallel rods or tubes distributed uniformly on a surface of a cylinder. Neighboring rod-shaped or tubular electrodes are supplied with the two phases of an RF voltage. This creates a pseudopotential between the rod-shaped or tubular electrodes, which keeps the ions in the interior of the cylinder. With two pairs of rod-shaped or tubular electrodes, a quadrupole field is created between the electrodes; with more than two pairs of rods, hexapole, octopole, decapole fields, etc. are created. The rod-shaped or tubular electrodes used to guide ions have a diameter of less than one millimeter and are typically 10 to 50 centimeters long. The interior formed by the electrodes is very narrow and has a diameter of only 2 to 4 millimeters, by means of which sufficiently strong multipole fields can be generated with low RF voltages.
Apart from these rod-shaped or tubular electrodes, other shapes of electrodes are described in DE 195 23 895 A1 and U.S. Pat. No. 5,572,035A with which an ion-guiding pseudopotential can be generated.
Nowadays, ion guides are used in almost all mass spectrometers in which the ions are generated outside the vacuum (out-of-vacuum ion sources), for example by ESI (electrospray ionization) or APCI (atmospheric pressure chemical ionization). The ion guides here often comprise several electrically isolated RF multipole segments of these rod-shaped or tubular electrodes, which can differ with respect to the number and arrangement of the electrodes, and the frequency and amplitude of the RF voltage, for example.
Some of the mass analyzers can only be operated under ultra-high vacuum conditions (p<10−6 Pa). In contrast, the out-of-vacuum ion sources are operated at up to atmospheric pressure. If generated out-of-vacuum, the ions are first transferred from the region of the ion source through an opening or capillary into the vacuum system and conveyed on to the mass analyzer. The residual gas originating from the ion source is evacuated in several differential pump stages until the operating pressure of the mass analyzer is reached. The chambers of adjacent differential pump stages are interconnected only via small openings. The rod-shaped or tubular electrodes are often limited to the chamber; the ion guide then consists of several RF multipole segments separated from each other.
For some types of mass analyzer, particularly for ion cyclotron resonance spectrometers (ICR MS), the ultra-high vacuum region can be separated from the ion source by means of a valve. Sliding valves, which have thicknesses of around 30 millimeters in the direction of the axis of the ion guide, are the preferred option here because they are small. Separation by means of a valve is necessary in order to protect the ultra-high vacuum in the mass analyzer from contamination when the ion source and adjacent regions of the ion guide are cleaned or serviced. The availability of the mass spectrometer is increased because the sensitive ultra-high vacuum of the mass analyzer is maintained during cleaning or servicing and does not have to be produced again in a protracted process. The insertion of a valve means that the two adjacent RF multipole segments of the ion guide have a separation which, when the valve is open, the ions can bridge only with a lens system, given the current prior art.
The method of operation of the ICR MS means that a strong magnetic field is required in the mass analyzer. The transfer of the ions from the region where there is no magnetic field into the strong magnetic field of the mass analyzer is demanding because the ions are reflected, as if in a magnetic bottle, at the magnetic field of the mass analyzer if they do not move close to the axis and parallel to the lines of the magnetic field. Outside the mass analyzer there is a magnetic stray field which can neither be completely avoided nor shielded sufficiently. The separating valve can modify the magnetic stray field in such a way that the valve has to be taken into consideration in the design of the lens system.
Ion guides are also used for types of ionization in which the ions are generated within the vacuum (in-vacuum ion source), such as matrix-assisted laser desorption and ionization MALDI. The ion sources which operate on the MALDI principle are used in ion trap mass spectrometers (IT MS), ion cyclotron resonance spectrometers (ICR MS) and time-of-flight mass spectrometers (TOF MS).
When using in-vacuum ion sources, the ion guides are used mainly in cases where the ions are not only guided but the ion guide also fulfils further objectives during the conditioning of the ions. These objectives consist in cooling the ions in a damping gas, in the dissociation of the ions by molecular collisions (CID=collision induced dissociation) or by electron capture (ECD=electron capture dissociation), in the intermediate storage of the ions, or in the selection in mass filters, for example. The differences in the objectives also result in the ion guide being subdivided into RF multipole segments because the individual RF multipole segments have different operating parameters. The most important operating parameters here are the number and arrangement of the electrodes, the frequency and the voltage amplitude of the RF voltage, additional DC voltage between and along the rod-shaped electrodes, and the pressure conditions in the interior between the electrodes. The operating parameters of an individual RF multipole segment are adapted to suit its specific objective, but are also determined by the mass spectrometer, comprising ion source, ion guide and mass analyzer.
The ions collide with the neutral molecules of a collision gas in a fragmentation cell and dissociate (CID). If the ions have low kinetic energy, the collisions in the gas do not lead to a fragmentation but only to a damping of the ion motion and cooling of the ions. The fragmentation or damping cells are often separated from the neighboring RF multipole segments in order to maintain the required vacuum conditions in the other RF multipole segments and in the mass analyzer. As is the case with the differential pump stages of an out-of-vacuum ion source, these gas-filled cells are only connected to the neighboring chambers via small openings and separate the RF multipole segments of the ion guide.
At the transition between the RF multipole segments of the ion guide, the fringing fields cause the ions at the ends of the RF multipole segments to be partially reflected, resulting in loss of transmission. These transmission losses during the passage between the RF multipole segments can be minimized by interposing diaphragms and lenses. An ion guide comprising a single RF multipole segment has lower losses and increases the sensitivity of the mass spectrometer.
If the diaphragms or lenses are put at a repelling DC potential for a certain period, then the pseudopotential of the RF multipole field and the DC potential of the diaphragms or lenses temporarily store the ions in the interior, which is defined by the rod-shaped or tubular electrodes and the diaphragms or lenses.
Mass spectrometers have been described in DE 196 29 134 C1 and DE 199 37 439 C1 which make it possible to choose between more than one ion source by sliding or turning movable RF multipole segments of the ion guide. It is therefore possible to change the configuration of the mass spectrometer without having to ventilate it. In both publications, an individual movable RF multipole segment has no electrical contact to other RF multipole segments of the ion guide. In order to avoid losses as the ions pass between the RF multipole segments of the ion guide, the distance between adjacent RF multipole segments must be as small as possible without causing electrical flashovers or crosstalk. Nevertheless, there are losses at the electric fringing fields between the RF multipole segments. In addition, each movable RF multipole segment of the ion guide must be individually connected to an RF voltage.